My mid-1980s Miyata 610 is the same, with no threads on the rear eyelets. I used some stainless bolts and nylock nuts I had lying around, and no wobble whatsoever after 5,000+ miles, many with heavy loads in the rear panniers. Rack is a Blackburn EX-1, I think. The rear fender has 2 stays on each side, and the attachment sandwich has the rack between the two fender stays.